The invention relates to micromechanical mirrors with a high-reflection coating for the deep-ultraviolet (DUV) and vacuum-ultraviolet (VUV) spectral range, based on a substrate, a highly reflective aluminium layer, at least one transparent blooming coating and at least one compensation layer for stress compensation. The invention likewise relates to a method for the production of such micromechanical mirrors with a high-reflection coating and also to the use thereof inter alia for the production of microsensors, optical data stores or video and data projection displays.
The production, modulation, conduction and detection of light presently occupy the central position in the field of optics research. The constant increase in modulation speeds and the trend towards higher integration densities are forcing miniaturisation of microoptic components and have led to the development of functional layers based on nanostructures for active optical components. The synergy of optoelectronics and micromechanics has led to a new class of integrated microoptoelectromechanical systems (MOEMS) with completely new fields of application and great potential for the near future. Examples of MOEMS applications are phase front corrections by adaptive optics, lab-on-chip and also telecommunications applications.
A completely new application which forms the background to this invention is UV lithography by means of spatial light modulators SLM, in which the productivity of conventional optical lithography and the high-resolution properties of electron beam lithography are combined (U. Ljungblad, U. Dauderstädt, P. Dürr, T. Sandström, H. Buhre, H. Lakner, “New laser pattern generator for DUV using spatial light modulator”, Microelectronic Engineering 57-58 (2001)23-29; T. Sandström, U. B. Ljungblad, P. Dürr, H. Lakner “High-performance laser pattern generation using spatial light modulators (SLM) and deep-UV radiation, Proceedings of SPIE Vol. 4343 (2001) 35; R. Thielsch, “Optical coatings for the DUV/VUV”, in Interference Coatings, by N. Kaiser and H. K. Pulker (Editors), Springer Series in Optical Sciences, Volume 88 (2003)). Applications of this technology are mask exposers for the production of photomasks for semiconductor technology or direct illumination systems for mask-free lithography which, relative to existing laser or electron-beam-based illumination systems, are significantly more economical and rapid. These systems operate in the DUV or VUV spectral range where, because of the strong absorption of most materials, beam damage plays an important role. The reduction in the absorption of reflecting microactuators, so-called micromirrors, or the achievement of high reflectances in the DUV/VUV (>90%) is therefore demanded urgently not only for reducing light losses but above all also for preventing degradation effects on micromirrors.
Although the technology of solid mirrors for the DUV/VUV range is extensively known, to date there are no optical silverings for micromirrors in the DUV/VUV which meet these requirements. The reason for this is the band conditions which in comparison with solid mirrors are more complex by a multiple and which silvering which is compatible with the technology of MOEMS must meet.
There are two fundamentally different concepts for the production of silvering: 1. dielectrical Bragg mirrors, i.e. stacks of layers with high or low reflective index; and 2. metallic mirrors.
In the literature, there have been presented specially for the DUV/VUV range, both Bragg mirrors (Zs. Czigany, M. Adamik, N. Kaiser, “248 nm laser interaction studies on LaF3/MgF2 optical coatings by cross-sectional transmission electron microscopy”, Thin Solid Films 312 (1998) 176-181; A. Gatto, J. Heber, N. Kaiser, D. Ristau, S. Günster, J. Kohlhaas, N. Marsi, M. Trovo, R. P. Walker, “High-performance DUV/VUV optics for the Storage Ring FEL at ELETTRA”, Nuclear Instruments and Methods in Physics Research A 483 (2002)357-362; N. Kaiser, H. K. Pulker (Editors), Optical Interference Coatings (Springer Series in Optical Sciences, 88), Springer-Press, Berlin, Heidelberg, N.Y., 2003) and Silvered Aluminium Mirrors. Integrated micromirror arrays with non-bloomed aluminium mirrors exist for applications in the visible spectral range (VIS) which are used in projection displays. Micromirror arrays for applications in UV (inter alia mask-free lithography at 248 nm) are manufactured by the Fraunhofer Institute for Photonic Microsystems, Dresden. A non-bloomed aluminium alloy is used here as material. Only a few works are involved with bloomed silverings for micromechanical actuators. The existing works are thereby restricted to the visible spectral range (400-800 nm). To date no works are known for bloomed micromirrors for the DUV/VUV.
The application of reflecting microactuators in the DUV and VUV (140-400 nm) requires coating thereof with high-reflection layer systems in order, on the one hand, to minimise the intensity loss by absorption and scattering and, on the other hand, to avoid the introduction of power into the actuator material and consequently caused structural instabilities, such as e.g. distortion of unsupported structures, but also oxidation or corrosion of the actuator materials. In contrast to the solid large-surface high-reflection (HR) mirrors, mirror layers on unsupported microactuators must meet additional requirements: the HR silvering must be able to be structured with methods of semiconductor technology, their reflectance not being permitted to be reduced by application of these methods. In order to ensure the flatness of the microactuators within a temperature interval around the operating point, the silvered actuators constructed in general from two and more layers must be compensated for with respect to layer tensions and thermal expansion.